Conventional cameras use lens systems comprised of substantially spherical lenses. General design principles are applied to reduce aberrations and to achieve a desired image quality. When less aberration is desired, more lens elements are added. Examples of general structural design principles used in conventional camera lenses include symmetry around the system aperture, combining large diameter lenses with small diameter lenses, combining low dispersion (Abbe number >50) and high dispersion lenses (Abbe number <50), use of substantially spherical lenses, use of thin substrates to bend the lenses and use of artificial vignetting.
Lenses used in wireless telephones (e.g., cellular telephones) present a unique challenge because they must be compact, inexpensive, used with a digital pixel array and provide a high-quality picture. To achieve these goals, designers have used wafer-level optics (WLO), which involves packaging small lenses with the digital circuitry, including a pixel array. Currently, however, lenses used in wafer-level optics are highly aspheric, or “free form,” often with aspheric coefficients of the 10th to 14th order, unlike lenses used in more traditional cameras.
FIG. 1 shows an example of a packaged lens system 101 used in a cell phone camera. Lens system 101 has an outer lens substrate 102 and an inner lens substrate 103, wherein inner lens substrate 103 is between outer lens substrate 102 and the image plane on a pixel array. Outer lens substrate 102 has one positive lens 107, and inner lens substrate 103 has two highly aspheric, or “free form” lenses 105, 106 separated by substrate 104. Outer lens substrate 102 may have a second lens as well. Lens system 101 includes a spacer 108 that is connected to inner lens substrate 103 and outer lens substrate 102 and fixes the lenses in the position shown in FIG. 1.
In current wafer-level lens designs such as the illustrated lens system 101, there is a lack of symmetry and thus strong ray bending (e.g., ray bundle W) to achieve a short total track of the imaging systems with a comparatively large focal length and small chief ray angle (CRA). Aberrations introduced by these strict specifications are reduced by introducing free form lenses 105, 106 with high aspheric coefficients. These free form lenses actually cause an increase in aberrations if, by the influence of fabrication tolerances, the ray bundles do not follow their expected path through the lenses. To reduce size, lens system 101 requires lens 106 be separated from the image plane by distance d, which also requires that lens 106 have a larger diameter than would be required if lens 106 were farther from the image plane. Moreover, fabrication of free form structures can be complicated.
Accordingly, a new fabrication method for lens systems and wafer-level lens systems is desired.